Thursday, April 21, 2011

Charting the Grid

A couple weeks ago I put up a post on Portland's grid, street widths and 19th century American city planning. With an apology in advance for all the statistics, I've put together a table to illustrate some of the numbers behind the grid form.

What the table shows in the first column is the amount of land, in acres, needed to provide equivalent amounts of building square footage for grids built with a variety of street widths and building heights, while always maintaining a 200x200 foot grid block (Portland's block dimensions) and 1:1 street width to height ratio (the assumption being that a benefit of wider rights-of-way is that buildings may rise taller without reducing the amount of natural light at street level). For example, a city of two-story buildings on 20-foot streets would need to be 2.6 times as large as a city of ten-story buildings on 100-foot streets to contain the same amount of total built square footage.

The second column shows the amount of land, out of the total acres, that is occupied by the right-of-way in each scenario. At the end of the chart I offer three examples of common American development forms for purposes of comparison.

This is similar to Floor Area Ratio, in a way, but I thought it was more helpful as a way to visualize intensity of land use over larger areas. Some of the results to me were fairly surprising. I'll post a few more thoughts of my own on this later, but I'd be interested to hear some reactions.

(A few notes: the South Philadelphia neighborhood is essentially an area of 2, 3 and 4-story buildings set on 30-foot streets. Sylvan Park is a typical gridded suburb of the early 20th century with wide streets and alleys behind each row of houses, inflating the amount devoted to roadway. The Reserve is a randomly chosen example of a cul-de-sac neighborhood of houses approximately twice as large as those in Philadelphia or the modest bungalows of Sylvan Park, but I did not adjust on a per-unit basis.)

18 comments:

The idea that street width and building height have a natural comfortable ratio is a valid one, but 1:1 is the wrong ratio. I would say it is more like 3:1, in other words, three stories for ten foot ROW, or 30 stories for 100 foot ROW.

Anonymous: I chose 1:1 for the sake of simplicity, it's not to be taken as a personal endorsement (although it is a New Urbanist favorite). The important thing's the ratios between the various scenarios. A 3:1 ratio, though, would probably magnify the advantages of narrow streets over the wide street option, taking into account construction costs and the low likelihood that the permitted building envelope in the wider-street scenarios would be fully occupied by buildings.

Yes, I actually did a run-through with 300x300-foot blocks, but the problem becomes, at that dimension and beyond, that it is difficult to use the space efficiently for buildings on narrow lots. In Philadelphia, for example, the larger squares have been bisected by alleys (essentially reducing blocks to 150x300, or 200x400) to allow greater numbers of rowhouses.

Ultimately, though, it does not change the ratios very much, although the wide/taller scenarios do fare better in terms of the density comparison as block sizes grow.

I may be interpreting this a bit wrong but here's what I gather from the table:

Holding "natural light" constant, having 90 foot wide streets compared to 10 foot wide streets increases the amount of unbuildable land in the city by about 11%. However, it saves nearly 5 times as much space outside the city.

So to sweet spot seems to be 50 foot ROW and 5 stories? This way you balance the total travel distance necessary across the entire city as well as proximity of locations within a neighborhood right?

I think the location of the sweet spot will vary depending on one's own preferences. There isn't really supposed to be a single "right answer" here. These can be taken as averages, too, despite the premise of a uniform grid, so the 50 foot/5 story option could be modified to provide a variety of street widths, e.g. five 40-foot streets alternating with 100-foot boulevards.

This is an exercise with some interesting conclusions, but I would not go directly from this to some actual design. For example, let's say you wanted very, very high density. Why? I don't know but let's say you did. You decided that it made sense to build 40-story residential towers, somewhat like Hong Kong.

Then, of course, you would have to decide the spacing and layout of these towers. It is a little silly to build very tall towers and then space them far apart, as the resulting density is actually not much different (could be less) than a typical 4-5 story neighborhood. Remember, we want maximum density.

Well, what kind of street ROW do you need? If you don't have automobiles, then you can do just fine with a 15-20 foot street width. However, if you build the buildings all the way to the property boundary at 40 stories, that would be a very tall and narrow slot.

One solution is to build in a ziggurat pattern. The building is 5 stories next to the street, then "shrinks" by perhaps 20 feet, then there are another ten stories, then more "shrinkage", then the final 25 stories in a freestanding tower. In other words, at street level the distance between buildings is 15 feet, at six stories the distance is 15+20+20 or 55 feet, and then at the tower level the distance between towers is 15+20+20+20+20 or 95 feet.

This "ziggurat" pattern is actually quite common, for example the Empire State Building in New York City.

So, I would decide on how wide you want the street to be, for its function as a pedestrian place with shops for example, and also how much density or how tall you want the buildings to be, and then work out a design that incorporates both elements.

From this you can also see that it makes a lot more sense to build in a "ziggurat" pattern with a street width of 15-20 feet and then a 5-story building "layer," than it does to just make the street wider for no particular purpose at all.

Anonymous: the point of the exercise is not to show how cities should be built, necessarily, but to show how the combination of narrow streets and low buildings can yield densities nearly as high as urban forms which seem much more dense.

If building heights were kept the same in each scenario, the narrow street option would enjoy a crushing advantage in terms of spatial efficiency against the others, but the resulting streetscapes would be plunged into darkness. This would quickly result in a public backlash against building height (as in New York in the 1910s and countless other places since then). There is a need to balance quality of life with the benefits of density.

Doesn't the data show that increasing heights generally are a more effective way of increasing density than narrowing streets?

Well, yes, the wide street/tall building combination is somewhat denser than the lower and narrower options (although it will require more paved roadway area).

But here are the problems with that:

1) A city built uniformly to that density will not be a pleasant, human-scaled place. Cars will tend to dominate the wide streets and pedestrians will be at a disadvantage. For livability reasons we might prefer low-rise/narrow streets even if it's less dense. It is helpful to know that the narrow street form is only slightly less dense than the taller option, though, especially in comparison to suburban forms.

2) If building heights are reduced to bring them closer to a human scale, the density advantanges of the wide-street form are lost, and the street loses its sense of enclosure.

3) Consider also that it's unlikely (for economic reasons) that all buildings could possibly reach the 10-story height necessary to confer the density advantage. By contrast, the narrow street option only requires two and three story buildings which are much cheaper and quicker to construct, making it much more likely to hit its density "target."

Central Paris has some of the highest densities of any city in the world, around 65,000 people per square mile. This is all built to a 4-6 story height with narrow streets generally of 15-25 feet.

There is really no reason to aspire to densities of significantly more than 65,000 per square mile. However, if for some reason you wanted to try 200,000 per square mile, you could. I would suggest 30-50 story buildings with streets of 15-20 feet. The buildings would be in a "ziggurat" pattern, somewhat like the Empire State Building in NYC, or the Burj Dubai. This would allow both tall buildings and very narrow streets, without the "narrow slot" effect and with adequate light and air at street level.

You can see that both examples, of Paris and also my hyper-dense example. are not contained in your model which presumes a 1:1 height/width ratio. That's why I say it is not a particularly useful model.

You can see that both examples, of Paris and also my hyper-dense example. are not contained in your model which presumes a 1:1 height/width ratio. That's why I say it is not a particularly useful model.

The model simply shows one possible ratio. It could be scaled up to 2:1 or 3:1, but the relationships between the options should remain the same, and that's what I was trying to get across. I deliberately omitted a measure of the amount of square feet or total units for each option since it wasn't particularly relevant to what I wanted to show.

Thanks for the interesting ziggurat drawing. I believe that is similar to the concept that has been used in downtown Vancouver in recent years, although with wider streets (of course) and larger setbacks on the upper floors. Shorter and/or smaller-footprint buildings can employ the same concept too, though, in the form of steeply-pitched roofs. I recall seeing houses in southern Germany that were three or four floors on the base with another three or four under the roof! Granted this was due more to heavy snow loads than preserving sunlight, but the effect was to increase building volume dramatically without blocking out much natural light up above (it looks nice, too).

One of the interesting results of doing the "ziggurat" model is that although the building height is about 6x (34 vs 6), the sf is only about double if you discount it a little bit for the space taken by elevators. In other words, even this attempt to maximize density does not give you a result that is all that much different than a simple Traditional City layout with narrow pedestrian streets and 4-6 story building height. Maybe that is why Paris is still among the most dense cities in the world.

In other words, even this attempt to maximize density does not give you a result that is all that much different than a simple Traditional City layout with narrow pedestrian streets and 4-6 story building height.

Interesting -- and I would think the same total square footage in a traditional form would cost far less to build, unless there's an extreme scarcity of land. I'd be curious, then, to hear your opinion of height limits. If a city were to decide a certain density were amply sufficient, projecting that density on the basis of 4-6 story buildings on narrow streets, would it be justified in restricting heights?

Certainly you can reach very high densities with the Traditional City form -- probably in excess of 100,000 people per square mile. I would say that there is no practical reason to attempt to exceed that, with a few rare exceptions like Hong Kong perhaps. Given ample human stupidity, a height restriction would be an excellent method.

However, people also like their skyscrapers, and I think it is possible to integrate skyscrapers into a city which has mostly a Traditional City layout. This would require considerable finesse, which is certainly in short supply these days. The easiest "hybrid" form is to actually keep the two rather separate, like oil and water. You would simply have a "skyscraper district," in which all the buildings are very tall (think Midtown Manhattan), and then a lowrise district, where 4-6 story heights prevail (think SoHo or Chinatown).

However, even with very tall buildings, much could be done to combine them with Traditional City elements at the ground level (for example my ziggurat model with 20 ft street width). It is not necessary to adopt a Manhattan model of 19th Century Hypertrophic large streets, or a 20th Century Hypertrophic model of surrounding the skyscrapers with Green Space, and making the roadways even bigger (Dubai).

Considering the real-world Portland, I believe the existing streets downtown vary from 60 to 80 feet in width (with the exception of Broadway). Usually the east-west streets are 60 feet, and the north-south streets are 80 feet. This is used for 3 or 4 lanes of travel/parking on each street, plus wide sidewalks with street trees.

This suggests that the current level of development in Portland (5 to 10 stories, with a few exceptions) is about at a 1:1 ratio of street width to building height, which surprises me. Walking around, it feels taller, perhaps because the scattered taller buildings are dominant visually.

To get streetscape more consistent with a traditional city (with a 2:1 height:width ratio), we would need to either narrow the streets to 30 or 40 feet, or allow taller buildings up to 12 to 20 stories. Seeing that the street pattern is hard to change, encouraging development up to a 20 story height limit (with perhaps a few taller buildings along Broadway or Glisan) would give the best results without losing too much light at street level.

This also suggests that Portland needs to expand its CBD to the margins of Downtown, and perhaps across the river to the eastern waterfront and the Lloyd district, if we are going to be able to keep jobs and people downtown as the region grows; there is only so much more space left downtown.